To solve the three dimensional structures of multiple murine T cell receptors specific for a viral octapeptide (VSV8) bound to the K b MHC class I molecule T cells recognize cell-associated pathogens of viral, bacteria[ or fungal origin as well as tumor antigens through a unique molecule termed the T cell receptor (TCR). The TCR complex consists of multiple transmembrane chains on the surface of T lymphocytes',". The disulfide-linked a-P heterodimer is a clonally unique component that possesses a recognition site for antigen in the context of the major histocompatibility complex (MHC), while the invariant CD3 components (y, 6, e, and Q are involved in signal transduction. Because of the intimate membrane association of this complex, study of the molecular features of TCR recognition necessarily have been indirect. Recent efforts to understand the nature of this recognition process have focused on structural studies of the system. It has been known for a few years that both class I and class 11 MHC molecules, despite the very different chemical compositions of the two subunits, have the same topologic structure"-s. in particular, the way MHC molecules present the antigenic peptide is similar. A short antigenic peptide (8-10 residues for class I and >1 2 residues for class 11) lies in a large groove at the top of the MHC molecule. This groove is composed of eight strands of anti-parallel P-sheet as a floor, and two anti-parallel a-helices as side walls. In class I MHC molecules, the whole groove is formed by domain al and a2 from the same a chain; in class 11 MHC molecules, the groove is formed by domain al, and 01 from two different chains (a and 0). Structural details of how peptides bind with high affinity to these MHC molecules have been well characterized.